A form builder that's simple to use, with fieldsets in a single file and Fenom syntax for MODX.

form.chunk.tpl

{var $forms = [
    'Schule' => [
        'fields' => [
            'Adresse' => [            
                'schule' => [
                    'type' => 'text',
                    'label' => 'Schule',
                    'required' => true,
                ],
                'strasse' => [
                    'type' => 'text',
                    'label' => 'Straße und Hausnummer',
                    'required' => true,
                ],
                'plz' => [
                    'type' => 'text',
                    'label' => 'PLZ',
                    'width' => 30,
                    'required' => true,
                ],
                'ort' => [
                    'type' => 'text',
                    'label' => 'Ort',
                    'width' => 60,
                    'required' => true,
                ],
            ],
            'Ansprechperson' => [
                'vorname' => [
                    'type' => 'text',
                    'label' => 'Vorname',
                    'width' => 40,
                    'required' => true,
                ],
                'nachname' => [
                    'type' => 'text',
                    'label' => 'Nachname',
                    'width' => 50,
                    'required' => true,
                ],
                'email' => [
                    'type' => 'text',
                    'label' => 'E-Mail',
                    'required' => true,
                    'validate' => ':email'
                ],
                'telefon' => [
                    'type' => 'text',
                    'label' => 'Telefon',
                ],
            ],
            'Nachricht an uns' => [
                'betreff' => [
                    'type' => 'text',
                    'label' => 'Betreff',
                    'required' => true,
                ],
                'nachricht' => [
                    'type' => 'textarea',
                    'label' => 'Nachricht',
                    'required' => true,
                ],
            ]
        ],
    ],
]}
{var $formKey = $.get.form ?: 'Schule'}
{var $fieldsValidate = []}
{var $emailTplFields = []}
{foreach $forms[$formKey]['fields'] as $title => $section}
    {foreach $section as $id => $field}
        {if $field['required']}
            {var $fieldsValidate[] = $id ~ ( $field['validate'] ?: '' ) ~ ':required'}
        {/if}
        {var $emailTplFields[] = '<li><strong>' ~ $field['label'] ~ ':</strong> ' ~ $.post[$id] ~ '</li>'}
    {/foreach}
{/foreach}

{var $emailTplFields = $emailTplFields | join : ''}

{'!formit' | snippet : [
    'hooks' => 'FormItSaveForm,email',
    'formName' => $formKey,
    'formEncrypt' => true,
    'emailFrom' => $_modx->config.emailsender,
    'emailTo' => $fieldContent.settings.recipient,
    'emailSubject' => 'Anfrage von Webseite: ' ~ $formKey,
    'emailTpl' => '@CODE:    
        <h2>Neue Anfrage: ' ~ $formKey ~ '</h2>
        <ul>
            ' ~ $emailTplFields ~ '
        </ul>
    ',
    'placeholderPrefix' => '',
    'clearFieldsOnSuccess' => true,
    'validate' => $fieldsValidate | join : ',',
    'validationErrorMessage' => 'Bitte füllen Sie die erforderlichen Felder aus.',
    'successMessage' => $fieldContent.settings.successMessage,
]}

<div class="col-full start-2-m end-8-m">

    <h2 id="form" class="f5 b mt4 mb3">Kontaktformular</h2>

    <ul class="list flex g1 f0 mb5">
        {foreach $forms as $id => $form}
            <li>
                <a
                    href="{$_modx->makeUrl($_modx->resource.id)}?form={$id}#form"
                    class="
                        btn btn--rounded
                        {if $id == $formKey}
                            btn--active
                        {/if}
                    "
                >{$id}</a>
            </li>
        {/foreach}
    </ul>

    <form
        action="{$.server.REQUEST_URI}#form"
        method="post"
        class="form"
        novalidate
    >
        {if $_modx->getPlaceholder('error_message') || $_modx->getPlaceholder('validation_error_message') || $_modx->getPlaceholder('success')}
            <div
                aria-live="assertive"
                role="status"
                class="
                    form__message b p4 mb5 br3 br--bl0

                    {if $_modx->getPlaceholder('success')}
                        bg-green-5
                    {else}
                        bg-red-5
                    {/if}
                "
            >
                {$_modx->getPlaceholder('validation_error_message')}
                {$_modx->getPlaceholder('error_message')}
                {$_modx->getPlaceholder('fi.successMessage')}
            </div>
        {/if}

        {foreach $forms[$formKey]['fields'] as $title => $section}
            <fieldset class="flex flex-wrap justify-between g2 mb4 mb5-m bn">
                <legend class="w-100 f4 b mt4 mt5-m mb3-m">{$title}</legend>
                {foreach $section as $id => $field}
                    <div class="form__item w-{$field['width'] ?: '100'}">
                    
                        {var $error = $_modx->getPlaceholder('error.' ~ $id)}

                        {if $field['type'] == 'text'}                    
                            <label for="{$id}" class="form__label f2">
                                {$field['label']}
                                {if !$field['required']}
                                    <span class="o-50">(optional)</span>
                                {/if}
                                {if $error}
                                    <div id="{$id}_error" class="f2 b red-5">{$error}</div>
                                {/if}
                            </label>
                            <div class="form__field form__field--input">
                                <input
                                    type="text"
                                    name="{$id}"
                                    id="{$id}"
                                    value="{$.post[$id]}"
                                    {if $field['required']}
                                        required
                                    {/if}
                                    {if $error}
                                        aria-invalid="true"
                                        aria-errormessage="{$id}_error"
                                    {/if}
                                />
                            </div>
                        {/if}

                        {if $field['type'] == 'textarea'}                    
                            <label for="{$id}" class="form__label">
                                {$field['label']}
                                {if !$field['required']}
                                    <span class="o-50">(optional)</span>
                                {/if}
                                {if $error}
                                    <div id="{$id}_error" class="f2 b red-5">{$error}</div>
                                {/if}
                            </label>
                            <div class="form__field form__field--textarea">
                                <textarea
                                    name="{$id}"
                                    id="{$id}"
                                    {if $field['required']}
                                        required
                                    {/if}
                                    {if $error}
                                        aria-invalid="true"
                                        aria-errormessage="{$id}_error"
                                    {/if}
                                >{$.post[$id]}</textarea>
                            </div>
                        {/if}

                    </div>
                {/foreach}
            </fieldset>
        {/foreach}

        <p class="f2 mv4-m sa11y-ignore">
            Wir schützen die übermittelten Daten. Alle Infos dazu in der <a href="{$_modx->makeUrl(23)}" target="_blank" class="bb">Datenschutzerklärung</a>.
        </p>

        <button
            type="submit"
            class="btn btn--clean btn--bubble btn--bubble-black"
        >
            <span>Senden</span>
            <span class="icon icon-font">
                {'arrow-right' | svg}
            </span>
        </button>

    </form>

</div>